1. Field of the Invention
The present invention relates generally to the field of two-wire, high frequency electronic ballasts for powering gas discharge lamps and the like and, more particularly, to a two-wire electronic ballast arrangement which achieves a unity power factor and greatly reduces power supply current harmonics in a simplified, low-cost manner.
2. Description of the Prior Art
Typical fluorescent lamps comprise a sealed cylinder of glass having a heating filament at either end and filled with a gas such as mercury vapor. The supplied voltage is utilized to heat the filaments to a point where a thermoionic emission occurs such that an arc can be struck across the tube causing the gas to radiate. Initial radiation given off by gases such as mercury vapor is of a short wavelength principally in the ultraviolet end of the spectrum and thus little visible light is produced. In order to overcome this problem, the inside of the tube is coated with a suitable phosphor which is activated by the ultraviolet radiation and, in turn, emits visible light of a color that is characteristic of the particular phosphor or mixture of phosphor employed to coat the tube.
Solid-state ballasts must provide the same primary function as the conventional core-coil ballasts well known in the art, i.e., they must start and operate the lamp safely. Solid-state ballasts normally convert conventional 60 Hz AC to DC and then invert the DC to drive the lamps at a much higher frequency. That frequency generally is in the 10 to 50 KHz range. It has been found that fluorescent lamps which are operated at these higher frequencies have a higher energy efficiency than those operated at 60 Hz, and they exhibit lower power losses. In addition, at high frequencies, annoying 60 cycle "flickering" and ballast hum are eliminated.
An important consideration in the operation of dimming ballast lamps is concerned with the fact that in order to sustain the arc across the lamps, the filament voltage must be maintained to a predetermined level. The maintenance of this predetermined voltage level in a low-cost scheme for dimming the output of the fluorescent tubes in a solid-state ballast system to produce an energy-saving, light-dimming arrangement has long been a problem in the art. One prior solution to this problem is illustrated and described in a co-pending application of Zoltan Zansky, the inventor in the present application, Ser. No. 210,650, filed Nov. 26, 1980, now U.S. Pat. No. 4,392,087 and assigned to the same assignee as the present application.
In the prior art the main power supply for solid-state ballasts has usually consisted of line current rectified by a rectifier bridge and filtered by inductive and/or capacitive means. One of the greatest problems associated with such a system concerns distortion in the rectified main power supply current which results in heavy contamination of the main power supply current with third, fifth or higher harmonics. This produces an inefficient power factor, shorter lamp life and may also result in overheating of the neutral wire of the building wiring which produces inefficiencies including power losses in the building transformer and other parts of the distribution power network. Such harmonics have been eliminated in the prior art by the use of a second stage converter or by using a large filtering inductor/capacitor circuit in the system. This, however, is quite expensive and still results in a considerable amount of power loss in the ballast circuit.
One example of such a prior art approach to the problem is illustrated and described in an article by Martin Gunther entitled, "Innovations for the Accessories for Light Sources: the electronic ballasts are coming" (title translated from the German), Licht, (pp. 414-416) 7-8/81. That reference depicts a solid-state ballast circuit in which a second stage converter is added ahead of the filter capacitor. This converter is a "boost-type" or a "flyback" converter, which has the characteristic of drawing pure sinusoidal current from the main power supply and in this manner eliminating the harmonic and associated power factor problems. While this prior art approach is effective in reducing harmonic distortion, the addition of the second converter stage increases the cost of the solid-state ballast substantially, and increases the system power loss and circuit heat generation.